Stable mortarless gravity facing walls derived from stackable interlocking overlapping concrete blocks or stretchers adapted to be anchored to the compacted mass of an embankment backfill by embankment penetrating anchoring components derived from interlocking concrete blocks called either tiebacks or headers are well known.
The control and stability of embankments achieved by retaining wall systems using interlocking concrete stretchers and tiebacks or headers are exemplified by Canadian patent no. 941,626, U.S. Pat. No. 4,490,075, and U.S. Pat. No. 4,815,897.
The footings for the installations of the aforementioned patents are prepared first through excavation of the soil to a depth of the order of 9 inches (230 mm) upon which 2 to 3 inches (50-75 mm) of granular base is compacted, whereupon the lowermost or first course of the concrete blocks or stretchers are laid out either at a selected angle in relation to the horizontal, as shown by Canadian patent no. 941,626, or levelled in all directions, as in the case of the aforementioned U.S. patents.
The footings may also include concrete pads laid lowermost upon or within the compacted granular base where deemed appropriate.
According to Canadian patent no. 941,626 where the height of the embankment to be contained, or the conditions of the soil of the embankment, or climatic conditions require that the gravity facing wall be strengthened, wider, heavier and thicker concrete blocks or stretchers can be provided in the lower courses of the facing wall. Further, by introducing tieback or header blocks at selected intervals in the facing wall, the increased weight or mass of such facing wall and increased penetration of the embankment by such tieback or header blocks impart greater resistance to the shifting or overturning forces generated by the soil pressures of the embankment and therefore provide greater overall stability and longevity to such an installation.
The modular stacked interlocking concrete blocks or stretchers of the gravity facing walls of the aforementioned U.S. Pat. Nos. 4,490,075 and 4,815,897, may not require any embankment penetrating anchoring components where the facing walls confining the embankment have sufficient weight or mass overall to resist dislodgement or overturning by the driving forces within the embankment; such driving forces result from stresses due to the weight of the soil, surcharge load, water pressures or the like.
Where soil conditions of the embankment or climatic conditions are more critical or the height of the embankment contained requires the gravity facing wall to be strengthened, reinforcement to strengthen and stabilize the gravity facing wall derived from modular stacked interlocking concrete blocks or stretchers arranged in horizontal courses of the aforementioned U.S. patents can be accomplished by introducing a series of embankment penetrating modular concrete blocks in the form of tiebacks or headers, which can be appropriately introduced into and interlock with the facing wall blocks or stretchers at selected intervals within the horizontal courses.
As well, such an installation may also include modular stacked interlocked overlapped rear blocks or stretchers which, together with the facing wall blocks or stretchers and tiebacks or headers, define an embankment penetrating cribbing structure to thereby more securely anchor the facing wall against dislodgement or overturning.
Such retaining wall systems utilizing concrete blocks for the facing wall as well as for embankment penetrating reinforcement have certain limitations in that different sizes and shapes of blocks must be manufactured and therefore different moulds provided which increase the costs of production.
Moreover with such systems, because of their overall weight or mass, transportation and labour costs for delivering and installing the units are quite substantial.
It is well known that unstable zones within the slopes of earthen or soil embankments can be strengthened, and thereby better controlled or stabilized, by appropriately anchored and tensioned horizontally layered sheets or strips of mesh or grid-like material, either metallic or synthetic, arranged within the embankment.
When so layered, anchored, and maintained in tension under compression through compacting of the earthen or soil fill of the embankment, the surfaces of the mesh or grid-like material frictionally engage with the soil or earthen particles and so reinforce or strengthen the mass in those regions whereby the likelihood of slope failure is substantially reduced.
Since by introducing within the embankment tensioned layered sheets or strips of mesh or grid-like material greater stability is imparted to the embankment, any facing wall to be installed to contain same would have less soil pressure exerted from behind, and, accordingly, thinner or lighter blocks could be utilized in the facing wall of such an installation.
In addition, by selectively tying or securing the layered sheets or strips to the facing wall to serve as an anchor for such wall, the tensioned sheets or strips would impart even greater stability to the installation.
Whereas retaining wall systems that utilize facing panels or blocks tied to horizontally layered sheets or strips of mesh or grid-like material and tensioned within compacted backfill are known and represented by the following U.S. patents, namely U.S. Pat. Nos. 3,925,994, 4,324,508, 4,661,023, 4,728,227, and 4,824,293, several of them are quite complex, utilizing various shapes of interlocking facing blocks or panels with fittings or accessories for establishing the tie or interconnection between such facing blocks or panels and the sheets or strips.
U.S. Pat. No. 3,925,994 discloses a relatively simple reinforcement wherein elongated earth-embedded comparatively wide net-like sheets fabricated from a substantially non-corrosive metallic or synthetic material have their opposed ends rigidly anchored between pairs of spaced-apart stacked interlocking concrete beam elements.
Such connections depend upon gravity clamping pressures exerted by the stacked interlocking concrete beam elements upon the sandwiched edges of the soil reinforcing sheets extending therebetween which reduces, if not eliminates, any direct pressure contact between the contiguous surfaces of the interlocking concrete beam elements thereby reducing frictional forces between such elements which resist the relative displacement thereof and reinforce the integrity of the structure.
Also, such sandwiched net-like sheets permit seepage or entry and the collection of ground waters between the contiguous surfaces of the interlocking concrete beam elements which would promote deterioration of the contiguous surfaces of such elements, and, particularly, under freezing and thawing temperatures, impart greater instability to the stacked structure of such interlocking concrete beam elements, rendering such structure more vulnerable to shifting or overturning as a result of soil pressures from behind, as well as to loosening of the ties or connections between the tensioned sheets and interlocking concrete beam elements.
A more complex system using anchoring grids derived from sheets or strips of synthetic material comprised of longitudinally extending ribs intersected by transversely extending bars or ribs to be tied to a facing wall panel in an embankment reinforcing structure is disclosed by U.S. Pat. No. 4,728,227.
According to the aforementioned patent a piece or section of the grid-like material must first be cast into the facing wall panel with a segment thereof or tab portion extending therebeyond to establish one element for the connection of the soil reinforcing grid to such facing wall panel. The connection between the exposed grid-like segment or tab portion and the soil reinforcing grid is established by matching the longitudinally extending ribs thereof and by drawing one set of ribs through the other, creating an elongated channel for the reception of a separate rod which is adapted to be gripped and held within the elongated channel under tensioning imparted first by instrumentation extending between the facing wall and the soil reinforcing grid and then maintained through compaction of the earth or soil fill deposited behind the wall panels.
Such a connection between a synthetic grid-like piece or section and a cast concrete panel is inherently weak and under tension tends to deteriorate. Particularly, ground waters can penetrate the facing wall panel at the intersections between the pieces or sections of the grid-like material where they project from the concrete panel which will erode and deteriorate such connection.